Certain devices such as wafer defect scanners, laser printers, document scanners, projectors and the like often employ a collimated laser beam that scans across a flat surface in a straight line path. These devices employ tilting mirrors to deflect the beam to perform the scanning. These tilting mirrors may be, or may include, Micro Electro Mechanical Systems (“MEMS”) devices.
Common mirrors used in MEMS devices, referred to herein as MEMS mirrors, include a stator and a rotor, with the rotor or structures carried by the rotor being reflective. The stator and/or rotor are driven with a drive signal which results in the rotor oscillating with respect to the stator, thereby changing the angle of reflectance of an incident light beam on the rotor. By oscillating the rotor between two orientations, an opening angle of the mirror is defined, and scanning of the light beam across the flat surface is accomplished.
If a MEMS mirror fails, such as when the rotor undesirably becomes stuck, the laser beam may shine in a fixed direction rather than be oscillating at a high frequency. Where a laser beam becomes fixed, particularly when the laser is of a high power, damage can result to surfaces that the laser shines onto. This damage can occur in a very short period of time, thus it is desirable to not only be able to detect when a MEMS mirror fails so that the laser beam can be switched off, but to detect that failure of the MEMS mirror as quickly as possible.